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Possible to use 1D and (N-1)D slice iteration with xt::xtensor? #2116

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AntoinePrv opened this issue Aug 19, 2020 · 3 comments · May be fixed by #2747
Open

Possible to use 1D and (N-1)D slice iteration with xt::xtensor? #2116

AntoinePrv opened this issue Aug 19, 2020 · 3 comments · May be fixed by #2747
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@AntoinePrv
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Description of the problem

I've tried using the examples of 1-D slice iteration and (N-1)-dimensional iteration by replacing xt::xarray<int> with xt::xtensor<int, 3> and found unexpected outcomes:

  • 1-D slice iteration (xt::axis_slice_begin1): compiles and run without errors, but all slice are empty (prints {}). Changing the axis does not change the outcome
  • (N-1)-dimensional iteration: does not compile, fails with 
    xtensor/xaxis_iterator.hpp:112:24: error: no matching constructor for
  initialization of 'shape_type' (aka 'array<unsigned long, 3UL>')
        shape_type shape(e_shape.size() - 1);

Desired outcome

Similar iteration as xt::xarray. Eventually with compile time check on the axis (given that shape is known).

Alternative considered

Casting back and forth to an xt::xarray. I am unsure of the performance impacts, if any.

System information

This is running xtensor 0.21.5 from conda-forge and compiling on MacOs with AppleClang 11.0.3
@tdegeus
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tdegeus commented Sep 7, 2020

That sounds like a bug. Was the only thing you changed with respect to the example the type of the array?

@AntoinePrv
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Yes, I recall making a proper test where I only changed the type of the array.

@bjmiao
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bjmiao commented Mar 5, 2021 •
edited

hi is there any update on this issue?
A similar thing is when I apply axis_end(xt, 0) to an xtensor, the compiler will complain:

/scratch/user/include/xtensor/xaxis_iterator.hpp: In instantiation of ‘auto xt::detail::derive_xstrided_view(CT&&, typename std::decay<_Tp>::type::size_type, typename std::decay<_Tp>::type::size_type) [with CT = const xt::xtensor_container<xt::uvector<float, std::allocator<float> >, 3, (xt::layout_type)1, xt::xtensor_expression_tag>&; typename std::decay<_Tp>::type::size_type = long unsigned int]’:
/scratch/user/include/xtensor/xaxis_iterator.hpp:172:49:   required from ‘xt::xaxis_iterator<CT>::xaxis_iterator(CTA&&, xt::xaxis_iterator<CT>::size_type, xt::xaxis_iterator<CT>::size_type, xt::xaxis_iterator<CT>::size_type) [with CTA = const xt::xtensor_container<xt::uvector<float, std::allocator<float> >, 3, (xt::layout_type)1, xt::xtensor_expression_tag>&; CT = const xt::xtensor_container<xt::uvector<float, std::allocator<float> >, 3, (xt::layout_type)1, xt::xtensor_expression_tag>&; xt::xaxis_iterator<CT>::size_type = long unsigned int]’
/scratch/user/include/xtensor/xaxis_iterator.hpp:329:16:   required from ‘auto xt::axis_end(E&&, typename std::decay<_Tp>::type::size_type) [with E = const xt::xtensor_container<xt::uvector<float, std::allocator<float> >, 3, (xt::layout_type)1, xt::xtensor_expression_tag>&; typename std::decay<_Tp>::type::size_type = long unsigned int]’
xtensor.cpp:66:34:   required from here
/scratch/user/include/xtensor/xaxis_iterator.hpp:111:24: error: no matching function for call to ‘std::array<long unsigned int, 3>::array(std::array<long unsigned int, 3>::size_type)’
             shape_type shape(e_shape.size() - 1);
                        ^~~~~
In file included from /usr/include/c++/8/tuple:39,
                 from /usr/include/c++/8/functional:54,
                 from /scratch/user/include/xtensor/xstrides.hpp:14,
                 from /scratch/user/include/xtensor/xaccessible.hpp:14,
                 from /scratch/user/include/xtensor.hpp:4,
                 from xtensor.cpp:6:
/usr/include/c++/8/array:94:12: note: candidate: ‘std::array<long unsigned int, 3>::array()’
     struct array
            ^~~~~
/usr/include/c++/8/array:94:12: note:   candidate expects 0 arguments, 1 provided
/usr/include/c++/8/array:94:12: note: candidate: ‘constexpr std::array<long unsigned int, 3>::array(const std::array<long unsigned int, 3>&)’
/usr/include/c++/8/array:94:12: note:   no known conversion for argument 1 from ‘std::array<long unsigned int, 3>::size_type’ {aka ‘long unsigned int’} to ‘const std::array<long unsigned int, 3>&’
/usr/include/c++/8/array:94:12: note: candidate: ‘constexpr std::array<long unsigned int, 3>::array(std::array<long unsigned int, 3>&&)’
/usr/include/c++/8/array:94:12: note:   no known conversion for argument 1 from ‘std::array<long unsigned int, 3>::size_type’ {aka ‘long unsigned int’} to ‘std::array<long unsigned int, 3>&&’
In file included from /scratch/user/include/xtensor.hpp:9,
                 from xtensor.cpp:6:
/scratch/user/include/xtensor/xaxis_iterator.hpp:116:26: error: no matching function for call to ‘std::array<long int, 3>::array(std::array<long int, 3>::size_type)’
             strides_type strides(e_strides.size() - 1);
                          ^~~~~~~
In file included from /usr/include/c++/8/tuple:39,
                 from /usr/include/c++/8/functional:54,
                 from /scratch/user/include/xtensor/xstrides.hpp:14,
                 from /scratch/user/include/xtensor/xaccessible.hpp:14,
                 from /scratch/user/include/xtensor.hpp:4,
                 from xtensor.cpp:6:
/usr/include/c++/8/array:94:12: note: candidate: ‘std::array<long int, 3>::array()’
     struct array
            ^~~~~
/usr/include/c++/8/array:94:12: note:   candidate expects 0 arguments, 1 provided
/usr/include/c++/8/array:94:12: note: candidate: ‘constexpr std::array<long int, 3>::array(const std::array<long int, 3>&)’
/usr/include/c++/8/array:94:12: note:   no known conversion for argument 1 from ‘std::array<long int, 3>::size_type’ {aka ‘long unsigned int’} to ‘const std::array<long int, 3>&’
/usr/include/c++/8/array:94:12: note: candidate: ‘constexpr std::array<long int, 3>::array(std::array<long int, 3>&&)’
/usr/include/c++/8/array:94:12: note:   no known conversion for argument 1 from ‘std::array<long int, 3>::size_type’ {aka ‘long unsigned int’} to ‘std::array<long int, 3>&&’
make: *** [Makefile:10: xtensor] Error 1

I guess this is somehow related to this issue

Stef-Sijben added a commit to Stef-Sijben/xtensor that referenced this issue Nov 22, 2023
@Stef-Sijben
Fix xaxis_*_iterator shape/stride (xtensor-stack#2116)
bb95ed0 
An `xaxis_slice_iterator` always refers to a 1d view, so just use an
array of size 1 for the shape and stride.
Another type would probably be more optimal in case of compile-time
fixed size (e.g. `xtensor_fixed`), but at least this is correct.

Always use runtime dimensionality `xaxis_iterator` shape and strides.
Other types would probably be more optimal in case of compile-time
fixed dimension and/or size (e.g. `xtensor`, `xtensor_fixed`), but at
least this is correct.
This was referenced Nov 22, 2023
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Fix xaxis_*_iterator shape and strides types Stef-Sijben/xtensor
4 participants
@JohanMabille @tdegeus @AntoinePrv @bjmiao